Process for impregnating electrical coils

ABSTRACT

A process for impregnating an electrical coil which comprises a winding comprising two or more layers consisting of more than one turn of an electrically conducting wire-form material with a polymer material which electrically insulates the individual turns of the wire-form material from one another with the aid of a thermally curable epoxy resin composition, which composition is solid at room temperature and comprises the following constituents: 
     (a) an epoxy resin which is solid at room temperature, selected from 
     (a1) polyglycidyl ethers based on novolaks; 
     (a2) diglycidyl ethers based on bisphenols, and 
     (a3) mixtures of more than one of components (a1) and (a2); 
     (b) a crosslinking agent for component (a); 
     (c) a suitable accelerator for the reaction of component (a) and component (b); 
     (d) from 15 to 70 percent by weight, based on the overall weight of the composition, of filler selected from 
     (d1) calcium carbonate, 
     (d2) quartz flour, 
     (d3) wollastonite whose particles have an average ratio of length to cross-section which is less than 5:1; 
     (d4) mixtures of components (d1), (d2) and (d3) and 
     (d5) mixtures of components (d1), (d2), (d3) and (d4) with other inorganic fillers; and which also preferably comprises 
     (e) from 10 to 60 percent by weight, based on the overall weight of the composition, of inorganic reinforcements in the form of acicular or fibrillar particles with a length of from 0.05 to 2.5 mm which have an average ratio of length to cross-section of 5:1 or more, 
     the overall amount of component (d) and (e) being not more than 80 percent by weight, based on the overall weight of the composition, inter alia does away with the need to use sacrificial moulds and reduces by a multiple the curing time required for completing the coating of the coils, reducing it to the region of a few minutes.

The present invention relates to a process for impregnating anelectrical coil, comprising a winding of two or more layers each ofwhich consists of more than one turn of an electrically conductingmaterial in wire form, with a polymer material which electricallyinsulates the individual turns of the wire-form material from oneanother, wherein the individual windings of the coil are enveloped withthe aid of a curable epoxy resin composition and this composition isthermally cured, and to particularly preferred curable compositions forcarrying out the impregnation.

From WO-A-96/01481 it is known to use curable epoxy resin compositionswhich are liquid at room temperature and comprise (a) a liquid bisphenolA diglycidyl ether, (b) a crosslinking agent for it, (c) a suitableaccelerator for the reaction of the liquid diglycidyl ether (a) with thecrosslinking agent (b), and (d) a filler material which, based oncomponent (a) and (b), comprises from 40 to 60 percent by weight ofcalcite and acicular, synthetic wollastonite for the impregnation and,if desired, for the coating of electrical coils comprising windingswhich in turn comprise a multiplicity of layers each of which consistsof a large number of turns of a thin, wire-form conductor material,examples being the secondary windings of transformers or ignition coils.The use of liquid epoxy resin systems such as those mentioned, althoughit enables such windings to be impregnated sufficiently, i.e. enablesthe epoxy material to penetrate between all of the turns of a winding,nevertheless still has some disadvantages. For instance, the liquidepoxy resin systems can only be formulated in two-pack form, which asfar as the user is concerned constitutes a not inconsiderable increasein processing complexity. In addition, the liquid impregnating systemsreferred to require the use of a sacrificial mould having the externalcontours desired for the finished coil. The coil to be potted has to beintroduced into this mould. Subsequently it is necessary, generallyfollowing its evacuation, to fill the mould with the liquid curableepoxy resin composition and to cure the composition in the mould. Aftercuring, the mould can no longer be removed and must therefore remainpermanently around the coil, as a sacrificial mould. A further highlysignificant disadvantage of the abovementioned liquid epoxy resincompositions, moreover, is their extremely long curing time, which ingeneral amounts to several hours and which, for the mass production ofcoils, necessitates the use of huge curing ovens.

The object of the present invention is inter alia to avoid theabovementioned disadvantages associated with the impregnation of coils,especially ignition coils, print transformers and flyback transformers.

It has now been found that such coils can even be impregnated usingpreferably fibre-reinforced, curable epoxy resin compositions beingsolid at room temperature, i.e. at a temperature from about 15 to 25°C., and having the specific composition defined below. A particularsurprise in this context is that such compositions are also able topenetrate sufficiently the secondary windings of customary ignitioncoils and transformers even if they comprise relatively large amounts ofa reinforcing material which consists of inorganic fibres with a lengthof up to about 2.5 mm. The use of these solid epoxy resin compositionsdoes away with the need to use a housing (sometimes referred to as "losthousing") and reduces by a multiple the curing time required forcompleting the coating of the coils, reducing it to the region of a fewminutes, generally not more than 15 minutes.

The present invention therefore provides in its widest sense a processfor impregnating an electrical coil which comprises a winding comprisingtwo or more layers consisting of more than one turn of an electricallyconducting wire-form material with a polymer material which electricallyinsulates the individual turns of the wire-form material from oneanother with the aid of a thermally curable epoxy resin composition,which composition is solid at room temperature and comprises thefollowing constituents:

(a) an epoxy resin which is solid at room temperature, selected from

(a1) polyglycidyl ethers based on novolaks;

(a2) diglycidyl ethers based on bisphenols, and

(a3) mixtures of more than one of components (a1) and (a2);

(b) a crosslinking agent for component (a);

(c) a suitable accelerator for the reaction of component (a) andcomponent (b);

(d) from 15 to 70 percent by weight, based on the overall weight of thecomposition, of filler selected from

(d1) calcium carbonate,

(d2) quartz flour,

(d3) wollastonite whose particles have an average ratio of length tocross-section which is less than 5:1;

(d4) mixtures of components (d1), (d2) and (d3) and

(d5) mixtures of components (d1), (d2), (d3) and (d4) with otherinorganic fillers.

By impregnation is meant in this application a treatment of theelectrical coil whereby the individual turns of at least one coilwinding are enveloped with the aid of the curable epoxy resincomposition and the composition is thermally cured. This treatment may,however, comprise the total encapsulation of parts of the coil or elseof the whole coil, including the coil former, with the curable epoxyresin composition.

In addition, the composition employed in the process of the inventionpreferably comprises:

(e) from 10 to 60 percent by weight, based on the overall weight of thecomposition, of inorganic reinforcements in the form of acicular orfibrillar particles with a length of from 0.05 to 2.5 mm which have anaverage ratio of length to cross-section of 5:1 or more,

the overall amount of component (d) and (e) being not more than 80percent by weight, based on the overall weight of the composition.

It is preferred to employ specially selected compositions in which thereinforcement (e) consists of acicular or fibrillar particles having alength of from 0.15 to 2.5 mm. The present invention additionallyprovides these compositions.

With particular preference, the reinforcement (e) comprises a glassfibre material, especially milled or chopped glass fibres having alength of from 0.05 to 2.5 mm and a diameter of preferably from 10 to 30μm, which are available in various forms and commercially. A reinforcingmaterial which has proven particularly suitable, for example, comprisesglass fibres with a length from 0.05 mm, in particular from 0.15 mm, to2 mm, in particular to 1 mm: for example, approximately 0.2 to 0.25 mm.

The novolak-based polyglycidyl ethers which are employed in the processof the invention and in the compositions of the invention as component(a1) are in particular:

(a1.1) polyglycidyl ethers based on epoxy-phenol novolaks,

(a1.2) polyglycidyl ethers based on epoxy-cresol novolaks, or

(a1.3) mixtures of epoxy novolaks of type (a1.1) and/or (a1.2).

Component (a2) preferably comprises diglycidyl ethers based on bisphenolA. These may, for example, be pre-extended (advanced) resins obtainableby reacting bisphenol A diglycidyl ether with a stoichiometric deficitof substances having two functional groups which are reactive withepoxide groups, in particular by reacting from 1.1 to 2 mol of bisphenolA diglycidyl ether with 1 mol of bisphenol A.

The crosslinking agent (b) is judiciously selected from:

(b1) dicyandiamide,

(b2) polycarboxylic anhydrides and

(b3) mixtures of more than one of components (b1) and/or (b2) and/or oneor more polyphenols.

If an organic carboxylic anhydride is employed as crosslinking agent itis preferably an aromatic or cycloaliphatic polycarboxylic anhydride,especially a corresponding dicarboxylic anhydride, such as phthalic,hexahydrophthalic or methyltetrahydrophthalic anhydride. Preference isgiven in particular to carboxylic anhydrides which are solid at roomtemperature.

If the crosslinking agent includes a certain proportion of polyphenols,this may be advantageous in terms of the processing of the impregnatingcompositions. Since, however, the presence of relatively large amountsof polyphenols may hinder the penetration of the curable compositioninto the winding and hence reduce the depth of impregnation, the upperlimit for the proportion of polyphenol in the crosslinking agent isguided by the depth of impregnation required.

Preferred polyphenols are:

phenol novolaks,

cresol novolaks and/or

mixtures of novolaks of the said types.

The crosslinking agent (b) is employed in the commonly used amounts; forexample, in amounts of up to 50 percent by weight, preferably up to 25percent by weight, based on the epoxy resin. Dicyandiamide is used withparticular preference in an amount of from 0.16 to 0.5 mol per epoxideequivalent of the composition, and in particular is added in amountssuch that in the compositions of the invention there is from 0.25 to0.35 mol of dicyandiamide per epoxide equivalent.

When dicyandiamide is employed as crosslinking agent the latency of theepoxy resin compositions of the invention is particularly good, meaningthat they can be stored, for example, without problems at roomtemperature (from about 15 to 25° C.).

As accelerators (d) it is possible to employ all customary substanceswhich the skilled worker knows accelerate the reaction of epoxy resinswith dicyandiamide or with polyphenols or, respectively, with organicanhydrides. Examples of accelerators for the reaction with dicyandiamideare alkali metal alcoholates, tertiary amines, especiallyhexamethylenetetramine, phosphines, such as triphenylphosphine,quaternary ammonium compounds, substituted ureas, such asN-(4-chlorophenyl)-N,N'-dimethylurea orN-(3-chloro-4-methylphenyl)-N,N'-dimethylurea, Mannich bases, such as2,4,6-tris(dimethylaminomethyl)phenol or2,4,6-tris(diethylaminomethyl)phenol, imidazole or imidazolederivatives, such as 2-phenylimidazole, 2-ethylimidazole,2-methylimidazole or benzimidazole, and complexes of BCl₃ and BF₃ withtertiary amines such as trimethylamine, octyldimethylamine,triethylamine, piperidine, pyridine or hexamethylenetetramine.Preference is given to imidazoles, especially 2-ethylimidazole. Examplesof accelerators for the reaction with polyphenols are tertiary amines,such as benzyldimethylamine, imidazoles, such as imidazole,2-phenylimidazole, 2-ethylimidazole, 2-methylimidazole or benzimidazole,or quaternary ammonium compounds. Examples of accelerators for thereaction with carboxylic anhydrides are tertiary amines and their salts,such as N-benzyldimethylamine or triethanolamine, Mannich bases, such asthose already mentioned above, imidazole and imidazole derivatives,quaternary ammonium salts, such as benzyltrimethylammonium chloride,phosphonium salts, such as tetraphenylphosphonium bromide, and alkalimetal alkoxides. The amount of accelerator is preferably from 1 to 30parts by weight per 100 parts by weight of catalyst, with particularpreference from 3 to 20 parts by weight and, in particular, from 5 to 12parts by weight.

The particle size distribution of the filler which forms component (d)of the compositions described is preferably from 0.1 to 200 μm, inparticular from 0.2 to 60 μm. The calcium carbonate employed as filleris preferably finely divided and essentially anhydrous calciumcarbonate. Examples of inorganic fillers other than calcium carbonate,quartz flour and wollastonite are kaolin, dolomite, barium sulfate,talc, mica, alumina or aluminium oxide trihydrate. Where mixtures ofcalcium carbonate, quartz flour and/or wollastonite with other fillersare used, the compositions comprise preferably at least 20 and, inparticular, at least 30 percent by weight of calcium carbonate, quartzflour and/or wollastonite. The other fillers can be present, forexample, in an amount of up to 40, preferably in an amount of up to 30,percent by weight. With particular preference, wollastonite is the solefiller present in the compositions employed in accordance with theinvention.

In some cases it is found favourable, moreover, for the compositionsemployed in accordance with the invention to comprise a toughener as afurther component (f). The toughener in this case is preferably employedin amounts of from 0.5 to 5, preferably in the range from about one totwo, percent by weight, based on the overall composition.

The toughener (f) is preferably selected from:

(f1) polysiloxane-modified epoxy resins and

(f2) block copolymers based on polydimethylsiloxanes and caprolactone oron polycaprolactones.

Tougheners of this kind are known to the skilled worker and areobtainable in various forms and commercially.

If a toughener of type (f1) is employed it is in particular apolysiloxane-modified diglycidyl ether based on bisphenol A.

The compositions used in accordance with the invention to impregnateelectrical coils may additionally comprise a mould release agent as afurther component (g), examples being Hoechst OP-Wachs® (partiallyhydrolysed montan wax), Hoechst-Wachs KSL® (montan wax), carnauba wax,calcium stearate or similar mould release agents, or mixtures of one ormore of the said substances.

The curable compositions employed in accordance with the invention mayadditionally comprise further customary additives, examples beingadhesion promoters for the fillers and the reinforcing materials, suchas, in particular, silane adhesion promoters, pigments, such as carbonblack, or flexibilizers,

In one specific embodiment of the process of the invention a curablecomposition is employed which comprises the following constituents ascomponents (a), (b) and (c):

(a2) one or more diglycidyl ethers based on bisphenols;

(b2) one or more polycarboxylic anhydrides;

(c) a suitable accelerator for the reaction of component (a2) andcomponent (b2).

In a particularly preferred embodiment of the process of the inventionthe composition used to coat coils comprises the following constituentsas components (a), (b) and (c):

(a) an epoxy resin which is solid at room temperature, selected from

(a1) polyglycidyl ethers based on novolaks;

(a2) diglycidyl ethers based on bisphenols, and preferably

(a3) mixtures of more than one of components (a1) and (a2);

(b) a crosslinking agent selected from

(b1) dicyandiamide and

(b4) mixtures of dicyandiamide and polyphenols (b2); and

(c) a suitable accelerator for the reaction of component (a) and thechosen component (b1) or (b4), for instance hexamethylenetetramine.

The latter embodiment of the process produces impregnating compositionshaving a particularly high Tg generally above 150° C. (determined by thetorsional vibration test of ISO 6721, heating rate 2° C./min).

In the case of preferred specific variants of the latter embodiment ofthe coating composition used in accordance with the invention, component(a) is a polyglycidyl ether based on a cresol novolak and/or thecrosslinking agent (b) is either dicyandiamide alone or a mixture ofdicyandiamide and polyphenols in which the polyphenols are, inparticular, phenol novolaks, cresol novolaks or mixtures of thesenovolaks.

The curable compositions employed in accordance with the invention canbe prepared and homogenized, for example, in a customary manner with theaid of known mixing apparatus, such as a ball mill, co-kneader, rollmill or extruder, with or without the melting of the compositionsfollowed, if required, by comminution of the melted material.Appropriate combinations of the abovementioned homogenization techniquescan of course also be employed. It is likewise possible, for example, todissolve and/or suspend the components of the curable compositions in anappropriate solvent and then to evaporate the solvent to leave thehomogenized composition in solid form.

For the impregnation of the coil, this coil is introduced into a mouldstraight away or following preliminary heating. Then the curablecomposition is introduced into the coil-containing mould and isthermally cured therein under pressure, preferably at a very low cavitypressure of, for example, not more than 100 MPa for a period of, forexample, up to a maximum of 10 minutes, preferably less than about 5minutes, preferably at temperatures in the range from 140 to 250° C., inparticular from 160 to 200° C. and, with great preference, in the regionof about 180° C. The time pressure profile in the course of filling, andthe maximum pressure, is dependent on the coil to be impregnated. Theparameters referred to can easily be determined experimentally by theskilled worker.

Before the curable composition is introduced, the mould is preferablyevacuated and the curable composition is introduced into thecoil-containing, evacuated mould. This is the case in particular for thechamber windings which are common at present.

The impregnation process of the invention is suitable, for example, forimpregnating coil windings having a winding density of up to 1000 turnsper mm², preferably up to 500 turns per mm², it being possible forexample to employ wire with diameters of down to 20 μm, preferably downto 50 μm, for the windings. The thickness of the overall windings inthis case can be, for example, up to 7.5 mm, preferably up to 5 mm. Suchwindings are found, for example, in ignition coils, print transformersand flyback transformers.

In the case of the impregnation process of the invention it is preferredto operate in accordance with the principle of a customary compressionmoulding, transfer moulding or injection moulding process.

If an injection moulding process is employed, it is preferred--owing tothe comparatively low melt viscosities of the curable compositionsemployed in accordance with the invention--to use a cylinder assemblywith a closeable injection nozzle, so that the nozzle can be closedwhen, for example, the mould is open. It may likewise be advantageous inthis case to use a screw which has a nonreturn valve.

EXAMPLES

FIG. 1 shows a diagram of the press tool with which the test coils inthe subsequent examples are impregnated.

FIG. 2 shows a longitudinal section through the coil former used in theexamples, and its dimensions.

GENERAL PROCEDURE FOR PREPARING IMPREGNATING COMPOSITIONS

Glass fibres and/or fillers are mixed with a silane adhesion promoterand are milled for 30 minutes with a ball mill. If a toughener is used,it is milled subsequently in the required amount with the filler for 20minutes, likewise in a ball mill. Finally, all of the components of thecurable compositions, with the exception of the glass fibres, are milledfor about 4.25 hours in a ball mill. The glass fibres are then added,and milling is continued for 45 minutes. The resulting powders are,finally, processed to granules using a compressor.

The impregnating compositions specified in the examples are preparedusing the additives, reinforcing materials and fillers characterizedbelow:

    ______________________________________                                        Commercial name                                                                              Chemical composition                                           ______________________________________                                        NYAD ® 200 Natural wollastonite (200 mesh)                                NYAD ® 325 Natural wollastonite (325 mesh)                                OMYA ® BSH Natural, surface-coated calcium carbonate                                     powder (grain size distribution 0.1-20 μm;                                 D.sub.50 = 2.4 μm)                                          Milled Glass ® 737BD                                                                     Milled glass fibres (average length =                          (Owens Corning)                                                                              225 μm, .O slashed. = 15-16 μm)                          Albidur ® EP 2240                                                                        Siloxane-modified bisphenol A epoxy                                           resin                                                          Hoechst OP ® wax 125U                                                                    Partially hydrolysed montan wax                                Hoechst KSL ® wax                                                                        Montan wax                                                     Silan A 187 ®                                                                            Silane adhesion promoter                                       Printex V ®                                                                              Carbon black                                                   ______________________________________                                    

The performance properties of the impregnating compositions aredetermined in every example unless specified otherwise with the aid ofthe measurement methods indicated below:

TI: Torsional vibration test in accordance with ISO 6721

Flexural strength: ISO 178

Flexural modulus: ISO 178

Impact strength: ISO 179/1

Linear thermal expansion coefficient α: DIN 53752 (temperature range20-80° C.)

Impregnating, determining the depth of impregnation, assessing thequality of impregnation:

In all of the examples the test coil is impregnated in accordance withthe principle of transfer moulding, using the test apparatus showndiagrammatically in FIG. 1. This apparatus comprises two parts (1) and(6) which can be separated from one another. The first part (1) has aninjection compartment (2) for receiving a tablet (11) consisting of theimpregnating composition of the invention, the transfer piston (3), thecavity (4) and a bore (5) for accommodating a temperature sensor. Thesecond part (6) comprises a core (7) for mounting the coil (10) which isto be impregnated, a device (8) for removing the finished impregnatedcoil from the core (7) and a connection (9) for evacuating the cavity(4). A coil (10) which has been preheated to about 110° C. and has thedimensions indicated in FIG. 2, and which in all of its chambers has awinding comprising copper wire with a diameter of 94 μm with a windingdensity of approximately 100 turns per mm², the thickness of the windingrising from the topmost to the bottommost chamber from about 3.5 toabout 5.5 mm, is introduced into the cavity (4) of the impregnating toot(1,6), which is heated at 180° C. The granular impregnating compositionis compressed into a tablet while still cold and then is heated to about70° C. using a high-frequency preheating device. The tablet (11)preheated in this way is introduced into the injection compartment (2),and a vacuum of about 35 mbar is applied to the cavity. Subsequently,the impregnating composition is transferred into the cavity (4) over aperiod of about 15 s with the aid of the piston (3) (injection pressurebetween 80 and 150 bar). The subsequent curing time is 5 minutes. Thecoated and impregnated coil is demoulded. The coil, having beenwithdrawn, is sawn in the lengthwise direction and polished. Amicroscope is used to measure the depth of impregnation achieved in eachcase, and the quality of impregnation is assessed visually. In thisassessment, impregnation is classified as "good" if more than 95 percentof the space between the wire turns of a winding has been filled withthe impregnating composition.

Crack resistance:

In order to determine the crack resistance a specimen is produced bycoating a sharp-edged rectangular metal plate measuring 60 mm in length,30 mm in width and 4 mm in thickness with the respective impregnatingcomposition, leaving an area of about 8 mm by 8 mm free at each of thefour corners of the metal plate, and the composition is cured at 170° C.to 180° C. The thickness of the coating is approximately 3 mm in thedirection of the long side and the broad side of the metal plate, andabout 2 mm perpendicular thereto. After demoulding, the specimen issubjected to the sequence of treatment steps indicated in the tablebelow, and, following each treatment step, is investigated for crackingin the coating composition. The classification in terms of crackingcorresponds to the last treatment step following which the specimenstill shows no cracking.

    ______________________________________                                        Step No cracking after                                                        ______________________________________                                        1    --                                                                       2    being left at room temperature for 1 hour after demoulding               3    being left at room temperature for 4 hours after demoulding              4    being left at room temperature for 24 hours after demoulding             5    Step 4 + cooling from room temperature to 0° C. in ice-water,          30                                                                            min at 0° C.                                                      6    Step 5 + 30 min at 100° C., cooling to 0° C. in                 ice-water, 30 min                                                             at 0° C.                                                          7    Step 6 + cooling from room temperature to -20° C., 15 min at           -20° C.                                                           8    Step 7 + cooling from room temperature to -40° C., 15 min at           -40° C.                                                           9    Step 8 + cooling from 100° C. to -40° C., 15 min at             -40° C.                                                           10   Step 9 + cooling from 100° C. to -60° C., 15 min at             -60° C.                                                           ______________________________________                                    

EXAMPLE 1

1 kilogram of an impregnating composition is produced and examined inaccordance with the general procedure described above; its compositionis as follows:

    ______________________________________                                        Component                Percent by weight                                    ______________________________________                                        Epoxy-cresol novolak (epoxide content 4.3 eq/kg;                                                       19.51                                                Kofler melting point 79° C.)                                           Advanced bisphenol A diglycidyl ether (epoxide                                                         12.68                                                content about 1.7 eg/kg; Kofler melting point                                 55-65° C.)                                                             Cresol-formaldehyde resin (melting point 97-104° C.)                                            0.99                                                 Dicyandiamide            2.7                                                  ZK 191.2-K granules (accelerator based on o-cresol                                                     0.2                                                  novolak and 2-ethylimidazole; Kofler melting point                            60-70° C.)                                                             NYAD 200                 46.79                                                Milled Glass 737 BD      15.0                                                 Hoechst OP wax 125U      1.7                                                  Silan A 187              0.23                                                 Printex V                0.2                                                  ______________________________________                                    

The impregnating composition has the following properties:

    ______________________________________                                        Property                                                                      ______________________________________                                        Depth of impregnation [mm]                                                                                3.5                                               Quality of impregnation   good                                                Tg [° C.]           170                                                Flexural strength [MPa]    110                                                Flexural modulus [MPa]    14600                                               Impact strength [kJ/m.sup.2 ]                                                                             6                                                 Cracking resistance         7                                                 Linear thermal expansion coefficient α[K.sup.-1 ]                                                 33 · 10.sup.-6                             ______________________________________                                    

EXAMPLE 2

1 kilogram of a further impregnating composition is produced inaccordance with the general procedure described above; its compositionis as follows:

    ______________________________________                                        Component               Percent by weight                                     ______________________________________                                        Epoxy-cresol novolak (epoxide content 4.3 eq/kg;                                                      33.0                                                  Kofler melting point 79° C.)                                           Dicyandiamide           4.0                                                   ZK 191.2-K granules (accelerator based on o-cresol                                                    0.2                                                   novolak and 2-ethylimidazole; Kofler melting point                            60-70° C.)                                                             NYAD 200                43.9                                                  Milled Glass 737 BD     15.0                                                  Albidur EP 2240         2.0                                                   Hoechst KSL wax         1.7                                                   Silan A 187             0.2                                                   ______________________________________                                    

This impregnating composition has the following properties:

    ______________________________________                                        Property                                                                      ______________________________________                                        Depth of impregnation [mm]                                                                                5.5                                               Quality of impregnation   good                                                Tg [° C.]           ≈200                                       Flexural strength [MPa]    135                                                Flexural modulus [MPa]    11600                                               Impact strength [kJ/m.sup.2 ]                                                                             7                                                 Linear thermal expansion coefficient α[K.sup.-1 ]                                                 33 · 10.sup.-6                             ______________________________________                                    

EXAMPLE 3

1 kilogram of an impregnating composition is produced in accordance withthe procedure described above; its composition is as follows:

    ______________________________________                                        Component              Percent by weight                                      ______________________________________                                        Advanced bisphenol A digiycidyl ether (epoxide                                                       31.7                                                   content about 1.7 eq/kg; Kofler melting point 55-                             65° C.)                                                                Phthalic anhydride     5.2                                                    BCl.sub.3 N(CH.sub.3).sub.3 complex                                                                  0.3                                                    NYAD 325               40.4                                                   OMYA BSH               19.0                                                   Albidur EP 2240        2.0                                                    Hoechst KSL wax        1.4                                                    ______________________________________                                    

This impregnating composition has the following properties:

    ______________________________________                                        Property                                                                      ______________________________________                                        Depth of impregnation [mm]                                                                               5.5                                                Quality of impregnation good                                                  Tg [° C.]        ≈120                                          ______________________________________                                    

A complete impregnation of good quality is also achieved with theimpregnating compositions referred to in the above examples ifimpregnation is carried out not of the test winding (Test) impregnatedin the examples but of, for example, the windings A, B and C set out inthe table below:

    ______________________________________                                                            Winding density                                                                           Winding thickness                             Winding                                                                              Wire diameter [μm]                                                                      [1/mm.sup.2 ]                                                                             [mm]                                          ______________________________________                                        Test   94           100         3.5-5.5                                       A      61           180         0.95                                          B      40           465         3.5                                           C      65           254         3                                             ______________________________________                                    

EXAMPLE 4

1 kilogram of an impregnating composition is produced and examined inaccordance with the general procedure described above; its compositionis as follows:

    ______________________________________                                        Component                Percent by weight                                    ______________________________________                                        Epoxy-cresol novolak (epoxide content 4.3 eq/kg;                                                       19.50                                                Kofler melting point 79° C.)                                           Advanced bisphenol A diglycidyl ether (epoxide                                                         12.68                                                content about 1.7 eg/kg; Kofler melting point 55-                             65° C.)                                                                Cresol formaldehyde resin (melting point 97-104° C.)                                            1.00                                                 Dicyandiamide            2.7                                                  Hexamethlenetetramine    0.2                                                  NYAD 200                 46.79                                                Milled Glass 737 BD      15.00                                                Hoechst OP wax 125U      1.7                                                  Silan A 187              0.23                                                 Printex V                0.2                                                  ______________________________________                                    

This impregnating composition has the following properties:

    ______________________________________                                        Property                                                                      ______________________________________                                        Quality of impregnation   good                                                Tg [° C.]           160                                                Flexural strength [MPa]    130                                                Flexural modules [MPa]    13500                                               Impact strength [kJ/m.sup.2 ]                                                                             7.5                                               Linear thermal expansion coefficient α[K.sup.-]                                                   31 · 10.sup.-6                             ______________________________________                                    

What is claimed is:
 1. A process for impregnating an electrical coilmade up of two or more layers consisting of more than one turn of anelectrically conducting material in wire form, which process comprisesimpregnating the coil with a thermally curable epoxy resin compositionthat is solid at room temperature and comprises(a) an epoxy resin whichis solid at room temperature, selected from(a1) polyglycidyl ethersbased on novolaks; (a2) diglycidyl ethers based on bisphenols, and (a3)mixtures of more than one of components (a1) and (a2); (b) acrosslinking agent for component (a); (c) a suitable accelerator for thereaction of component (a) and component (b); (d) from 15 to 70 percentby weight, based on the overall weight of the composition, of fillerselected from(d1) calcium carbonate, (d2) quartz flour, (d3)wollastonite whose particles have an average ratio of length tocross-section which is less than 5:1; (d4) mixtures of components (d1),(d2) and (d3) and (d5) mixtures of components (d1), (d2), (d3) and (d4)with other inorganic fillers,wherein the epoxy resin compositionelectrically insulates the individual turns of the material in wire formfrom one another.
 2. A process according to claim 1, where thecomposition additionally comprises:(e) from 10 to 60 percent by weight,based on the overall weight of the composition, of inorganicreinforcements in the form of acicular or fibrillar particles with alength of from 0.05 to 2.5 mm which have an average ratio of length tocross-section of 5:1 or more,the overall amount of component (d) and (e)being not more than 80 percent by weight, based on the overall weight ofthe composition.
 3. A process according to claim 2, where thereinforcement (e) consists of acicular or fibrillar particles with alength of from 0.15 to 2.5 mm.
 4. A process according to claim 1, wherethe reinforcement (e) is a glass fibre material.
 5. A process accordingto claim 1, where component(a1) is selected from:(a1.1) polyglycidylethers based on epoxy-phenol novolaks, (a1.2) polyglycidyl ethers basedon epoxy-cresol novolaks and (a1.3) mixtures of components (a1.1) and(a1.2).
 6. Process according to claim 1, where component (a2) is adiglycidyl ether based on bisphenol A.
 7. A process according to claim1, where the crosslinking agent(b) is selected from:(b1) dicyandiamide,(b2) polycarboxylic anhydrides and (b3) mixtures of more than one ofcomponents (b1) and/or (b2) and/or one or more polyphenols.
 8. A processaccording to claim 7, where the polyphenols are selected from:phenolnovolaks, cresol novolaks and mixtures of such novolaks.
 9. A processaccording to claim 1, where the filler is wollastonite.
 10. A processaccording to claim 9, where the composition additionally comprises(f) atoughener.
 11. A process according to claim 10, where the toughener(f)is selected from:(f1) polysiloxane-modified epoxy resins and (f2) blockcopolymers based on polydimethylsiloxanes and caprolactone or onpolycaprolactones.
 12. A process according to claim 11, where thetoughener (f1) is a polysiloxane-modified diglycidyl ether based onbisphenol A.
 13. A process according to claim 1, where the compositioncomprises the following constituents as components (a), (b) and (c):(a2)one or more diglycidyl ethers based on bisphenols; (b2) one or morepolycarboxylic anhydrides; (c) a suitable accelerator for the reactionof component (a2) and component (b2).
 14. A process according to claim1, where the composition comprises the following constituents ascomponents (a), (b) and (c):(a) an epoxy resin which is solid at roomtemperature, selected from(a1) polyglycidyl ethers based on novolaks;(a2) diglycidyl ethers based on bisphenols, and preferably (a3) mixturesof more than one of components (a1) and (a2); (b) a crosslinking agentselected from(b1) dicyandiamide and (b4) mixtures of dicyandiamide andpolyphenols; and (c) a suitable accelerator for the reaction ofcomponent (a) and the chosen component (b1) or (b4), preferablyhexamethylenetetramine.
 15. A process according to claim 14, wherecomponent (a) is a polyglycidyl ether based on a cresol novolak.
 16. Aprocess according to claim 14, where the crosslinking agent (b) isdicyandiamide.
 17. A process according to claim 14, where thecrosslinking agent (b) is a mixture of dicyandiamide and polyphenols andthe polyphenols are selected fromphenol novolaks, cresol novolaks andmixtures of such novolaks.
 18. A process according to claim 1, where thecomposition additionally comprises(g) a mould release agent.
 19. Aprocess according to claim 1, where the coil is introduced into a mouldand then the curable composition is introduced into the coil-containingmould and is thermally cured therein under pressure.
 20. A processaccording to claim 19, where the mould is evacuated before the curablecomposition is introduced and the curable composition is introduced intothe coil-containing, evacuated mould.
 21. A process according to claim 1in accordance with the principle of the compression moulding, transfermoulding or injection moulding process.
 22. An injection mouldingprocess according to claim 21, where a cylinder assembly is used whoseinjection nozzle is closeable.
 23. An injection moulding processaccording to claim 20, where a screw is used which has a non-returnvalve.